Zwitterionic Bicyclobutane: An Intermediate in the Course of a Nucleophilic Vinylic-like Substitution Reaction on 3-Halobicyclobutanecarbonitri1e

In the nucleophilic reaction of the alkoxides MeO⁻, EtO⁻, i-PrO⁻, and CF₃CH₂O⁻ with 3-halobicyclobutanecarbonitrile (1-Cl, -Br), the ketal 3,3-dialkoxycyclobutanecarbonitrile (2) together with small amounts of the vinylic compound 3 were obtained. Rate constants for the reaction of 1 with MeO⁻, EtO⁻, and i-PrO⁻ in the parent alcohols were determined at 25 °C. In the reaction of MeO⁻ in MeOH, the reversible formation of the iminomethoxy compound 4 was also observed. The element effect k_cl/Br~ 4 indicates that in the first step of the reaction the nucleophile cleaves the central bond of 1 rather than the bond to the leaving group. In this step, a cyano-stabilized carbanion is formed on C-1 and a halo ether on C-3 of the molecule. Evidence is presented to show that unlike the addition-elimination mechanism in nucleophilic vinylic substitution reactions, there is no ring closure coupled with the expulsion of the nucleofuge to give the alkoxybicyclobutane 5. Instead, the halo ether decomposes to give oxocarbenium paired with both the carbanion on C-1 and the halide ion (path b, Scheme I). In the reactions of 1 with CF₃CH₂O⁻ in t-BuOH, the (2,2,2-trifluoroethoxy)carbenium is captured by a trifluoroethoxy ion to give 2 (R = CF₃CH₂) and by t-BuOH (which is otherwise unreactive) to give the two cis-trans isomers of the mixed ketal 6. In these reactions, the less stable isomer 6t is obtained preferentially over the more stable one 6c. This behavior is typical in cases where protonation of the cyclobutanic carbanion is the stereochemically controlling step. The ratio 2 (R = CF₃CH₂):6 is significantly affected by the halogen identity (Cl or Br), whereas there is no element effect on the cis-trans partition of the two isomers of 6. These last two observations together with analysis of rate constant ratios for similar reactions strongly support a reaction pathway in which the nucleofuge departs to give a zwitterionic intermediate before the carbanion is protonated or nucleophilically displaces the nucleofuge to form the covalent alkoxybicyclobutane 5. The zwitterionic species is in fact a bicyclobutane derivative in which the central bond is ionic rather than covalent. The origin of the barrier for the transformation of this ionic bond to a covalent one is discussed.